Dr. Peter McKenna:

There is not always diagnostic certainty about this. Dr. Murphy has also said this. If a mother was on a high dose of Epilim and a child was born with an obvious physical abnormality such as spina bifida, then it is entirely reasonable to conclude that one has led to the other. If, however, a mother was exposed to a low does for a short period of time during the pregnancy and the child subsequently developed a behavioural disorder, it is much more difficult to attribute the subsequent behavioural disorder with absolute certainty and clarity to the small dose of sodium valproate to which the child was exposed for a short period of time. This situation is not like many diagnoses that are absolute, where one can say "Yes" or "No". There are degrees of greyness and subtlety here that will need to be borne in mind.

A possible explanation for the difference between the 400 and the 40 is that the 400 might refer to the number of women who took the medication during the pregnancy and whose babies were exposed to it, while the 40 refers to the number who have received the absolute diagnosis. I speculate when I say this but it would appear to be the most reasonable explanation for the big disparity between the 400 and the 40.